Changing landscapes: Effects of varying refuge structure on the behavioral ecology and resistance management of western corn rootworm, Diabrotica virgifera virgifera LeConte

The western corn rootworm (Diabrotica virgifera virgifera LeConte) is a notorious, long-term pest of corn in the Midwest, having developed resistance to many pesticides as well as the cultural control practice of crop rotation. After nearly a century of research, the western corn rootworm is still abundant and difficult to control; resulting in millions of dollars in losses and some of the highest per acre pesticide use in the United States. Transgenic, or Bt corn, which contains a Bt toxin originally isolated from Bacillus thuringiensis, provides effective rootworm control and reduces pesticide use. However, due to the adaptable nature of the pest, resistance management is necessary in order to extend the durability of this management tactic. Resistance management is currently accomplished, according to the guidelines established by the Environmental Protection Agency (EPA), using the refuge strategy. A refuge, or section of corn plants that are not resistant to rootworm larvae, is planted with the Bt corn in one of several approved patterns. In order for this strategy to be effective, the refuge must provide a large reservoir of susceptible insects that mate with any potentially resistant insects surviving on Bt corn. To optimize refuge design and performance, a strong understanding of western corn rootworm biology and mating behavior in the Bt corn cropping system is crucial. This research examined: (1) the impact of refuge structure on western corn rootworm emergence timing and root damage, (2) the influence of natal host plants on adult head capsule width, (3) the composition and fate of the spermatophore, and (4) differences in the cuticular hydrocarbons of western corn rootworm based on several factors, including mating status and natal host (i.e., refuge or Bt corn). Three refuge structures (i.e., block, strip or seed mix) at two different sizes (10 or 20%) were evaluated, using rootworm emergence rates, in field plots over two seasons. The results indicated that western corn rootworm emergence from refuge and Bt corn plants was more synchronous in a seed mix refuge compared to the structured refuge treatments. This synchrony was best explained by larval movement between refuge and Bt corn plants, which was evidenced by an increase in root damage on Bt corn roots, and a decrease in root damage on refuge plants when refuge and Bt corn plants were in close proximity to each other. When the head capsule widths of beetles emerging from different host plants were compared, males emerging from Bt corn had significantly smaller head capsules. Western corn rootworm spermatophores, analyzed using a Bradford assay for proteins and an anthrone assay for sugar content, were found to contain relatively high levels of protein as well as minor amounts of sugar. The proteins were traced using the stable isotope, 15N, and the results indicated that the nitrogen received from males in the spermatophore is incorporated into the eggs by the female. Male head capsule width, pronotum width, and elytral length were found to have a significant positive relationship with spermatophore size. Cuticular hydrocarbons of western corn rootworms were sampled using solid-phase microextraction and were then analyzed using gas chromatography-mass spectrometry. Hydrocarbon profiles were compared on the basis of mating status, natal host (i.e., refuge or Bt corn) and sex. Some significant quantitative differences were identified between males that developed on different host plants, as well as between males and females. The synchrony of western corn rootworm emergence from refuge and Bt corn in seed mix refuge structures is advantageous for resistance management because it promotes mating between beetles emerging from refuge and Bt corn plants. Nevertheless, the larval movement between refuge and Bt corn, inferred from root damage data, is concerning because it could result in sublethal exposure to Bt toxins (as evidenced by the reduced head capsule width of males emerging from Bt corn), which may partially offset resistance management efforts. However, the diminished head capsule width of males from Bt corn may actually assist resistance management if males with larger head capsules (i.e., those from the refuge) have greater mating success and provide females with larger spermatophores. Overall, these results are favorable for the adoption of seed mix refuge structures as a resistance management strategy for western corn rootworm